2.0 Analysis 2.1 Cause of the Crankcase Explosion The scoring and the material deposited on the No. 8 liner, and the scuffing of the piston crown depict conditions that would be consistent with a breakdown of the cylinder liner lubrication film. This breakdown would have resulted in metal-to-metal contact between the piston rings and the liner with consequential hot spots on the liner. Further, the combustion gas blow-by can also ignite hydrocarbon vapours. Thus, these hot spots and/or combustion gas blow-by would likely cause ignition of the lubricating oil vapour/mist, resulting in the crankcase explosion. Prompt action by the second engineer in shutting down the main engine likely averted a more serious secondary explosion. 2.2 Final Lubricating Oil Safety Strainer Failure The final lubricating oil safety strainer is thought to have failed after the 03 March occurrence as no evidence of filter material was noted when the No. 7 bottom-end bearing was opened up at that time. The precise time of the disintegration of the safety oil strainer is not known. However, this information is not considered significant because it did not influence the outcome of this occurrence. 2.2.1 Lubricating Oil and Main Engine Wear As the main engine components are made of different metals, the presence of high levels of certain metals in a lubricating oil sample can be associated with engine component wear. For example, the presence of copper in the oil could indicate bearing wear; chromium, piston ring wear; iron, liner and piston ring wear; and aluminium may be introduced through piston wear. The elevated levels of these four elements in the lubricating oil sample are consistent with the wear found in the engine. As the No. 8 piston pin had been seized in the connecting rod bearing bush, it would indicate that the supply of lubricating oil to the bearing bush had been interrupted during operation. 2.3 Piston Crown Deposits When the piston crown deposits came in contact with the cylinder lubrication, an abrasive compound was formed, which would have contributed to the cylinder liner wear. 2.4 Monitoring of Wear Rate and Preventive Action Periodic cylinder liner inspections were carried out and the liner wears were recorded. From this, the liner wear rate per 1,000 hours could have been derived. This information, when used in conjunction with other available information, e.g., the higher than normal consumption of lubricating oil, would have provided an overview of the overall condition of the engine. In this instance, an oil sample analysis in December 1992 indicated an abnormal level of chromium (piston ring material) and, as abnormal liner wear rates were observed in some units during their past liner inspections, further investigation to determine the cause of the problem would have been in order. During the course of the investigation, no evidence or record was presented to suggest that such action had been initiated until after the 03 March explosion. 2.5 Employment and Monitoring Practices - Training and Safety A close monitoring of the main engine maintenance program by the company would have been in order since main engine maintenance is essential for the efficient, economic and safe operation of the vessel, there was a frequent turnaround of personnel, and there was no specific company training program in place. However, as abnormal wear rates of the main engine components had continued for a period of time without effective action, it is apparent that the company did not effectively monitor the condition of the main engine. 3.0 Conclusions 3.1 Findings The substantial wear on the cylinder liners can be attributed to the breakdown/destruction of the lubricating oil film because of the abrasive action of the deposits of combustion, metallic debris from the broken piston rings and worn liner material. The piston ring failures were most probably a result of excessive ring groove clearance. The top piston ring in unit No. 4 had been installed upside down. The lubricating oil sample taken before the occurrence revealed abnormal levels of chromium but no evidence or record was found to suggest that follow-up action had been initiated. Abnormal cylinder liner wear rates were found in six out of the nine units that had been overhauled since 1991. The bearing shell surfaces were scored by metal from the final lubricating oil safety strainer which disintegrated in the crankcase explosion. The breakdown of the liner lubricating oil film in the No. 8 unit resulted in hot spots on the liner. The hot spots in the No. 8 unit and/or the combustion gas blow-by resulted in the ignition of the main engine crankcase oil vapour/mist. Prompt action by the second engineer in shutting down the main engine likely prevented a more serious secondary explosion. 3.2 Causes The crankcase explosion in the main engine of the IRVING NORDIC was caused most likely by the ignition of the crankcase oil vapour/mist as a result of hot spots in way of the No. 8 cylinder liner and/or combustion gas blow-by. The primary contributing factor to this occurrence was the substandard condition of the main engine. 4.0 Safety Action 4.1 Action Taken Following the accident, the main engine was completely overhauled. In September 1993, the vessel was dry-docked and the main engine was again examined for excessive wear. Subsequently, the main engine piston crowns were modified in accordance with the manufacturer's recommendations. During the main engine overhaul, the company changed the grade of the fuel oil to maintain a clear running engine.